1. Field of the Invention
This invention relates to a demodulator which demodulates a signal modulated by differential phase shift keying.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows an example of the conventional demodulator arranged to demodulate a two-phase signal modulated by differential phase shift keying (hereinafter referred to as DPSK modulated signal). FIG. 2 is a timing chart showing the wave forms of signals obtained at various parts of FIG. 1. Referring to FIG. 1, the illustration includes an input terminal 1 arranged to receive the DPSK modulated signal; a shift register 2; an exclusive OR circuit (hereinafter referred to as EXOR) 3; a low-pass filter (hereinafter referred to as LPF) 4; a buffer amplifier 5; and an output terminal 6 arranged to produce a DPSK demodulated signal.
As shown at a part (a) of FIG. 2, the DPSK modulated signal which is received at the terminal 1 is of a wave form whereby data of one bit is transmitted every period of time T. The signal (a) thus received is read into a shift register 2 according to a clock pulse signal (f) received at another input terminal 7.
Assuming that the period of the clock pulse signal (f) is t, the number N of bits of the shift register 2 is arranged to be N=T/t(T&gt;t). The DPSK modulated signal is thus arranged to be delayed by the shift register 2 as much as the period T. The output signal (b) of the shift register 2 which is as shown at a part (b) of FIG. 2 and the incoming DPSK modulated signal (a) are supplied to the EXOR 3, which produces its output as shown at a part (c) of FIG. 2. The signal (c) thus produced from the EXOR 3 is processed through the LPF 4 and becomes a demodulated signal which is as shown at a part (d) of FIG. 2. The wave form of the demodulated signal (d) is shaped into a DPSK demodulated signal as shown in a part (e) of FIG. 2.
However, in accordance with the conventional demodulation method, it is hardly possible to obtain a correct demodulated signal in the event that a part of the DPSK modulated signal is affected by a drop-out during transmission or by distorted transmission resulting in a wave form part X as shown in FIG. 2. In that event, the transmitted data might be mistaken depending on the timing for taking in the demodulated data. To avoid the adverse effect of such a drop-out or distorted transmission, it is conceivable to lower the cut-off frequency of the LPF 4. That methods, however, makes it impossible to shorten a data transmission period T required per bit and hinders high speed transmission of data. Further, in the event of demodulation of a DPSK modulated signal in which the transmission period of time per bit of data is varying, it becomes extremely difficult to determine the cut-off frequency of the LPF. Then, in that event, the probability of demodulating wrong data increases.